<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }
        .pic{
            width: 100%;
        }
        .pic img{
            display: block;
            width: 100%;
        }
        .top{
            width: 80px;
            height: 80px;
            background-color: red;
            color: #fff;
            /* 固定定位是相对浏览器窗口可显示区域的定位，不管写在哪里 */
            position: fixed;
            /*层级顺序*/
            z-index: 10;
            bottom: 20px;
            right: 20px;
            
        }
    </style>
</head>
<body>
    <div class="pic">
        <img src="./img/固定定位长图背景.jpg" alt="">
    </div>
    <a href="#" class="top">top</a>
</body>
</html>